Reproducing objective for video disks

ABSTRACT

A reproducing objective for video disks comprising a first, second, third and fourth lenses in the order from the reproducing light source side, the first lens being a biconvex or plano-convex lens, the second lens being a negative meniscus lens, the third lens being a biconvex or plano-convex lens, the fourth lens being a positive meniscus lens, at least one of these lenses being arranged as a plastic lens, the reproducing objective being extremely light in weight and having a long working distance and high resolving power.

BACKGROUND OF THE INVENTION

(a) Field of the invention

The present invention relates to a reproducing objective for videodisks.

(b) Description of the prior art

For reproduction of signals recorded on a video disk, the method to makethe objective perform fine oscillation is often adopted in order tocorrect the tracking error as well as the method to oscillates agalvano-mirror. In this method, the objective performs fine oscillationof about 10 to 90 Hz. Therefore, the objective should be small in sizeand light in weight as far as possible. Besides, the objective should belocated at least at a certain distance from the video disk surface.Otherwise, the objective will contact the disk surface in actual use andwill cause damage to the disk. Therefore, the objective for video disksshould have a long working distance.

However, in case of known objective for video disks comprising lensesmade of optical glass, it is impossible to satisfy the requirements tomake the objective small in size and light in weight though it ispossible to make the working distance long. Especially, the requirementto make the objective small in size is not satisfied. To solve theabove-mentioned problem, objectives for video disks comprising lensesmade of plastics (plastic lenses) are also known. However, in case ofthose objectives, the working distance is short and insufficient and,moreover, the resolving power is lower compared with objectives forvideo disks comprising lenses made of optical glass though it ispossible to make the objective satisfactorily light in weight.

SUMMARY OF THE INVENTION

It is, therefore, a primary object of the present invention to provide ahigh-performance reproducing objective for video disks in which at leastone of lenses constituting the objective is arranged as a plastic lensin order to satisfy the requirement to make the objective light inweight and, moreover, for which the working distance is made long andresolving power is made high to the same degree as those of knownobjectives for video disks comprising lenses made of optical glass.

The reproducing objective for video disks according to the presentinvention has four-element lens configuration as shown in FIG. 1, i.e.,it comprises a first, second, third and fourth lenses in the order fromthe reproducing light source side. The first lens is a biconvex orplano-convex lens, the second lens is a negative meniscus lens, thethird lens is a biconvex or plano-convex lens and the fourth lens is apositive meniscus lens. Out of the above-mentioned lenses, at least onelens is arranged as a plastic lens. Besides, the reproducing objectivefor video disks according to the present invention satisfies thefollowing conditions (1) through (6):

(1) 2.1f<f₁ <3.2f

(2) -55f<f₂ <-3.9f

(3) 1.3f<f₄ <2.4f

(4) f<r₈ <2.3f

(5) d₂ <0.1f

(6) 0.5f<d₄

wherein reference symbols f₁, f₂ and f₄ respectively represent focallengths of the first, second and fourth lenses, reference symbol r₈represents the radius of curvature of surface on the video disk side ofthe fourth lens, reference symbol d₂ represents the airspace between thefirst and second lenses, reference symbol d₄ represents the airspacebetween the second and third lenses, and reference symbol f representsthe focal length of the lens system as a whole.

Meanings of the above-mentioned conditions are described below.

The condition (1) relates to spherical aberration to be caused by thefirst lens. When it is intended to correct astigmatism caused by thethird and fourth lenses so that astigmatism of the lens system as awhole becomes favourable, the degree of freedom concerning the shape ofthe first lens is limited to some extent. As a result, sphericalaberration is caused by the first lens. The condition (1) is to correctthe above-mentioned spherical aberration. When f₁ in the condition (1)becomes smaller than 2.1f, negative spherical aberration to be caused bythe first lens becomes too large and it is impossible to favourablecorrect it by the other lenses. When f₁ becomes larger than 3.2f, itbecomes impossible to favourably correct astigmatism to be caused by thethird and fourth lenses even when any value is selected for the focallength f₂ of the second lens. Moreover, when f₁ is larger than 3.2f,negative spherical aberration to be caused by the first lens is toolsmall and, therefore, spherical aberration of the lens system as a wholeincluding spherical aberration to be caused by the other lenses will beovercorrected.

The condition (2) is established for the purpose of making the secondlens cause positive spherical aberration of adequate value in order tocorrect negative spherical aberration caused by the first lens. When f₂in the condition (2) becomes smaller than -55f, positive sphericalaberration to be caused by the second lens becomes too small and, as aresult, spherical aberration of the lens system as a whole will beundercorrected. When f₂ becomes larger than -3.9f, spherical aberrationof the lens system as a whole will be overcorrected in conversely to theabove.

The condition (3) is established for the purpose of correctingastigmatism favourably. Astigmatism will be undercorrected when f₄ inthe condition (3) becomes smaller than 1.3f and will be overcorrectedwhen f₄ becomes larger than 2.4f and neither case is desirable.

The condition (4) is established for the purpose of favourablycorrecting astigmatism like the condition (3). To make the workingdistance long, it is desirable to make the radius of curvature ofsurface on the video disk side of the fourth lens small. That is, theworking distance becomes longer when r₈ is made smaller. When, however,r₈ is made too small, astigmatism will be overcorrected. The lower limitf for r₈ is to define the limit so that the above-mentioned radius ofcurvature will not be made too small. When r₈ becomes larger than 2.3f,astigmatism will be undercorrected.

The condition (5) relates to coma. When d₂ in the condition (5) becomeslarger than 0.1f, it becomes difficult to correct coma favourably. Whend₂ is made smaller, it is desirable for the manufacture of lens becauseit is then possible to make the shape of the first lens more convenientfor the manufacture.

As for the condition (6), it is possible to make the lens system smallerwhen d₄ is smaller. When, however, d₄ becomes smaller than 0.5f, comaand curvature of field will be caused to the degree that it will beimpossible to correct them by varying the radii of curvature of lenssurfaces. From the view point for making the working distance long, itis more advantageous when d₄ is larger.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a sectional view of the reproducing objective for videodisks according to the present invention; and

FIGS. 2 through 15 respectively show graphs illustrating aberrationcurves of Embodiments 1 through 14 of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the reproducing objective for video disksaccording to the present invention which satisfy the above-mentionedrespective conditions are as shown below.

    ______________________________________                                        Embodiment 1                                                                  NA = 0.45     WD = 0.534   β = -1/18.5                                   r.sub.1 = 3.0032                                                              d.sub.1 = 0.2322                                                                            n.sub.1 = 1.59119                                                                          ν.sub.1 = 57.8                                  r.sub.2 = -2.6218                                                             d.sub.2 = 0.0580                                                              r.sub.3 = -1.1963                                                             d.sub.3 = 0.3250                                                                            n.sub.2 = 1.49819                                                                          ν.sub.2 = 57.8                                  r.sub.4 = -2.4894                                                             d.sub.4 = 0.7545                                                              r.sub.5 = 2.1883                                                              d.sub.5 = 0.2322                                                                            n.sub.3 = 1.59119                                                                          ν.sub.3 = 57.8                                  r.sub.6 = -3.1827                                                             d.sub.6 = 0.0116                                                              r.sub.7 = 0.7015                                                              d.sub.7 = 0.1857                                                                            n.sub.4 = 1.59119                                                                          ν.sub.4 = 57.8                                  r.sub.8 = 2.1531                                                              t = 0.15      n.sub.t = 1.50                                                  f.sub.1 = 2.405                                                                             f.sub.2 = -5.044                                                f.sub.3 = 2.229                                                                             f.sub. 4 = 1.680                                                f = 1                                                                         Embodiment 2                                                                  NA = 0.45     WD = 0.460   β = -1/18.5                                   r.sub.1 = 2.0547                                                              d.sub.1 = 0.2300                                                                            n.sub.1 = 1.4902                                                                           ν.sub.1 = 57.8                                  r.sub.2 = -2.2681                                                             d.sub.2 = 0.0575                                                              r.sub.3 = -1.2145                                                             d.sub.3 = 0.3220                                                                            n.sub.2 = 1.4902                                                                           ν.sub.2 = 57.8                                  r.sub.4 = -2.2457                                                             d.sub.4 = 0.8165                                                              r.sub.5 = 1.2677                                                              d.sub.5 = 0.2300                                                                            n.sub.3 = 1.4902                                                                           ν.sub.3 = 57.8                                  r.sub.6 = -3.7974                                                             d.sub.6 = 0.0115                                                              r.sub.7 = 0.5727                                                              d.sub.7 = 0.1380                                                                            n.sub.4 = 1.4902                                                                           ν.sub.4 = 57.8                                  r.sub.8 = 1.7107                                                              t = 0.15      n.sub.t = 1.50                                                  f.sub.1 = 2.238                                                                             f.sub.2 = -6.013                                                f.sub.3 =  1.968                                                                            f.sub.4 = 1.689                                                 f = 1                                                                         Embodiment 3                                                                  NA = 0.45     WD = 0.459   β = -1/18.5                                   r.sub.1 = 2.3631                                                              d.sub.1 = 0.2296                                                                            n.sub.1 = 1.49819                                                                          ν.sub.1 = 57.8                                  r.sub.2 = -2.6102                                                             d.sub.2 = 0.0919                                                              r.sub.3 = -1.1435                                                             d.sub.3 = 0.3215                                                                            n.sub.2 = 1.49819                                                                          ν.sub.2 = 57.8                                  r.sub.4 = -1.7398                                                             d.sub.4 = 0.9186                                                              r.sub.5 = 1.1498                                                              d.sub.5 = 0.2067                                                                            n.sub.3 = 1.49819                                                                          ν.sub.3 = 57.8                                  r.sub.6 = -3.8750                                                             d.sub.6 = 0.0115                                                              r.sub.7 = 0.5679                                                              d.sub.7 = 0.1378                                                                            n.sub.4 = 1.49819                                                                          ν.sub.4 = 57.8                                  r.sub.8 = 1.4233                                                              t = 0.15      n.sub.t = 1.50                                                  f.sub.f.sub.2 = -8.160                                                        f.sub.3 = 1.805                                                                             f.sub.4 = 1.800                                                 f =  1                                                                        Embodiment 4                                                                  NA = 0.45     WD = 0.460   β = -1/18.5                                   r.sub.1 = 2.2811                                                              d.sub.1 = 0.2302                                                                            n.sub.1 = 1.49819                                                                          ν.sub.1 = 57.8                                  r.sub.2 = -2.6068                                                             d.sub.2 = 0.0575                                                              r.sub.3 = -1.1867                                                             d.sub.3 = 0.3222                                                                            n.sub.2 = 1.49819                                                                          ν.sub.2 = 57.8                                  r.sub.4 = -1.8608                                                             d.sub.4 = 0.8977                                                              r.sub.5 = 1.1534                                                              d.sub.5 = 0.2302                                                                            n.sub.3 = 1.49819                                                                          ν.sub.3 = 57.8                                  r.sub.6 = -3.9686                                                             d.sub.6 = 0.0115                                                              r.sub.7 = 0.5731                                                              d.sub.7 = 0.1381                                                                            n.sub.4 = 1.49819                                                                          ν.sub.4 = 57.8                                  r.sub.8 = 1.4170                                                              t = 0.15      n.sub.t = 1.50                                                  f.sub.1 = 2.481                                                                             f.sub.2 = -7.818                                                f.sub.3 = 1.821                                                                             f.sub.4 = 1.832                                                 f = 1                                                                         Embodiment 5                                                                  NA = 0.45     WD = 0.460   β = -1/18.5                                   r.sub.1 = 2.3727                                                              d.sub.1 = 0.2298                                                                            n.sub.1 = 1.49819                                                                          ν.sub.1 = 57.8                                  r.sub.2 = -2.5701                                                             d.sub.2 = 0.919                                                               r.sub.3 = -1.1415                                                             d.sub.3 = 0.3218                                                                            n.sub.2 = 1.49819                                                                          ν.sub.2 = 57.8                                  r.sub.4 = -1.7673                                                             d.sub.4 = 0.8963                                                              r.sub.5 = 1.1733                                                              d.sub.5 = 0.2298                                                                            n.sub.3 = 1.49819                                                                          ν.sub.3 = 57.8                                  r.sub.6 = -3.7409                                                             d.sub.6 = 0.0115                                                              r.sub.7 = 0.5716                                                              d.sub.7 = 0.1379                                                                            n.sub.4 = 1.49819                                                                          ν.sub.4 = 57.8                                  r.sub.8 = 1.4758                                                              t = 0.15      n.sub.t = 1.50                                                  f.sub.1 = 2.515                                                                             f.sub.2 = -7.805                                                f.sub.3 = 1.821                                                                             f.sub.4 =  1.782                                                f = 1                                                                         Embodiment 6                                                                  NA = 0.45     WD = 0.578   β = -1/18.5                                   r.sub.1 = 3.7269                                                              d.sub.1 = 0.2312                                                                            n.sub.1 = 1.49819                                                                          ν.sub.1 = 57.8                                  r.sub.2 = -2.3833                                                             d.sub.2 = 0.0925                                                              r.sub.3 = -1.0641                                                             d.sub.3 = 0.2081                                                                            n.sub.2 = 1.49819                                                                          ν.sub.2 = 57.8                                  r.sub.4 = -1.6543                                                             d.sub.4 = 0.9249                                                              r.sub.5 = 1.5397                                                              d.sub.5 = 0.2081                                                                            n.sub.3 = 1.49819                                                                          ν.sub.3 = 57.8                                  r.sub.6 = -3.1801                                                             d.sub.6 = 0.0116                                                              r.sub.7 = 0.7601                                                              d.sub.7 = 0.1387                                                                            n.sub.4 = 1.72309                                                                          ν.sub.4 = 28.46                                 r.sub.8 = 1.8463                                                              t = 0.15      n.sub.t = 1.50                                                  f.sub.1 = 2.955                                                                             f.sub.2 = -6.782                                                f.sub.3 = 2.113                                                                             f.sub.4  = 1.696                                                f = 1                                                                         Embodiment 7                                                                  NA = 0.45     WD = 0.580   β = -1/18.5                                   r.sub.1 = 3.9264                                                              d.sub.1 = 0.2321                                                                            n.sub.1 = 1.49819                                                                          ν.sub.1 = 57.8                                  r.sub.2 = -2.4314                                                             d.sub.2 = 0.0928                                                              r.sub.3 = -1.0479                                                             d.sub.3 = 0.2089                                                                            n.sub.2 = 1.49819                                                                          ν.sub.2 = 57.8                                  r.sub.4 = -1.6446                                                             d.sub.4 = 0.8704                                                              r.sub.5 = 1.6841                                                              d.sub.5 = 0.2321                                                                            n.sub.3 = 1.49819                                                                          ν.sub.3 = 57.8                                  r.sub.6 = -2.9405                                                             d.sub.6 = 0.116                                                               r.sub.7 = 0.8141                                                              d.sub.7 = 0.1741                                                                            n.sub.4 = 1.77861                                                                          ν.sub.4 = 25.71                                 r.sub.8 = 1.8989                                                              t = 0.15      n.sub.t = 1.50                                                  f.sub.1 = 0.3051                                                                            f.sub.2 = -6.561                                                f.sub.3 = 2.186                                                                             f.sub. 4 = 1.710                                                f = 1                                                                         Embodiment 8                                                                  NA = 0.45     WD = 0.530   β = -1/18.5                                   r.sub.1 = 2.7135                                                              d.sub.1 = 0.2304                                                                            n.sub.1 = 1.49819                                                                          ν.sub.1 = 57.8                                  r.sub.2 = -2.3475                                                             d.sub.2 = 0.0922                                                              r.sub.3 = -1.1732                                                             d.sub.3 = 0.2074                                                                            n.sub.2 = 1.49819                                                                          ν.sub.2 = 57.8                                  r.sub.4 = -2.2729                                                             d.sub.4 = 0.9102                                                              r.sub.5 = 2.3475                                                              d.sub.5 = 0.2304                                                                            n.sub.3 = 1.49819                                                                          ν.sub.3 = 57.8                                  r.sub.6 = 2.7135                                                              d.sub.6 = 0.0115                                                              r.sub.7 = 0.7232                                                              d.sub.7 = 0.1728                                                                            n.sub.4 = 1.77861                                                                          ν.sub.4 = 25.71                                 r.sub.8 = 1.8951                                                              t = 0.15      n.sub.t = 1.50                                                  f.sub.1 = 2.565                                                                             f.sub.2 = -5.193                                                f.sub.3 = 2.565                                                                             f.sub.4 = 1.411                                                 f = 1                                                                         Embodiment 9                                                                  NA = 0.45     WD = 0.531   β = -1/18.5                                   r.sub.1 = 2.4705                                                              d.sub.1 = 0.2306                                                                            n.sub.1 = 1.49819                                                                          ν.sub.1 = 57.8                                  r.sub.2 = -2.3810                                                             d.sub.2 = 0.9022                                                              r.sub.3 = -1.1846                                                             d.sub.3 = 0.2075                                                                            n.sub.2 = 1.49819                                                                          ν.sub.2 = 57.8                                  r.sub.4 = -2.4965                                                             d.sub.4 = 0.8647                                                              r.sub.5 = 2.3810                                                              d.sub.5 = 0.2306                                                                            n.sub.3 = 1.49819                                                                          ν.sub.3 = 57.8                                  r.sub.6 = -2.4705                                                             d.sub.6 = 0.0115                                                              r.sub.7 = 0.7225                                                              d.sub.7 = 0.1729                                                                            n.sub.4 = 1.77861                                                                          ν.sub.4 = 25.71                                 r.sub.8 = 1.7879                                                              t = 0.15      n.sub.t = 1.50                                                  f.sub.1 = 2.473                                                                             f.sub.2 = -4.776                                                f.sub.3 =  2.473                                                                            f.sub.4 = 1.454                                                 f = 1                                                                         Embodiment 10                                                                 NA = 0.45     WD = 0.531   β = -1/18.5                                   r.sub.1 = 2.3500                                                              d.sub.1 = 0.2306                                                                            n.sub.1 = 1.49819                                                                          ν.sub.1 = 57.8                                  r.sub.2 = -2.4361                                                             d.sub.2 = 0.0923                                                              r.sub.3 = -1.1911                                                             d.sub.3 = 0.2076                                                                            n.sub.2 = 1.49819                                                                          ν.sub.2 = 57.8                                  r.sub.4 = -2.6000                                                             d.sub.4 = 0.8418                                                              r.sub.5 = 2.3500                                                              d.sub.5 = 0.2306                                                                            n.sub.3 = 1.49819                                                                          ν.sub.3 = 57.8                                  r.sub.6 = -2.4361                                                             d.sub.6 = 0.0115                                                              r.sub.7 = 0.7255                                                              d.sub.7 = 0.1730                                                                            n.sub.4 = 1.77861                                                                          ν.sub.4 = 25.71                                 r.sub.8 = 1.7739                                                              t = 0.15      n.sub.t = 1.50                                                  f.sub.1 = 2.440                                                                             f.sub.2 = -4.639                                                f.sub.3 = 2.440                                                                             f.sub.4 = 1.470                                                 f = 1                                                                         Embodiment 11                                                                 NA = 0.45     WD = 0.519   β = -1/18.5                                   r.sub.1 = 2.0414                                                              d.sub.1 = 0.2306                                                                            n.sub.1 = 1.49819                                                                          ν.sub.1 = 57.8                                  r.sub.2 = -2.4915                                                             d.sub.2 = 0.0922                                                              r.sub.3 = -1.2311                                                             d.sub.3 = 0.2079                                                                            n.sub.2 = 1.49819                                                                          ν.sub.2 = 57.8                                  r.sub.4 = -3.2939                                                             d.sub.4 = 0.7955                                                              r.sub.5 = 2.0414                                                              d.sub.5 = 0.2306                                                                            n.sub.3 = 1.49819                                                                          ν.sub.3 = 57.8                                  r.sub.6 = -2.4915                                                             d.sub.6 = 0.0115                                                              r.sub.7 = 0.7239                                                              d.sub.7 = 0.1729                                                                            n.sub.4 = 1.77861                                                                          ν.sub.4 = 25.71                                 r.sub.8 = 1.6673                                                              t = 0.15      n.sub.t = 1.50                                                  f.sub.1 = 2.291                                                                             f.sub.2 = -4.083                                                f.sub.3 = 2.291                                                                             f.sub.4 = 1.521                                                 f = 1                                                                         Embodiment 12                                                                 NA = 0.45     WD = 0.466   β = -1/18.5                                   r.sub.1 = 1.4004                                                              d.sub.1 = 0.3362                                                                            n.sub.1 = 1.49819                                                                          ν.sub.1 = 57.8                                  r.sub.2 = ∞                                                             d.sub.2 = 0.0815                                                              r.sub.3 = -1.1768                                                             d.sub.3 = 0.6990                                                                            n.sub.2 = 1.49819                                                                          ν.sub.2 = 57.8                                  r.sub.4 = -1.4754                                                             d.sub.4 = 0.5296                                                              r.sub.5 = 1.4004                                                              d.sub.5 = 0.3362                                                                            n.sub.3 = 1.49819                                                                          ν.sub.3 = 57.8                                  r.sub.6 = ∞                                                             d.sub.6 = 0.0125                                                              r.sub.7 = 0.5856                                                              d.sub.7 = 0.1398                                                                            n.sub.4 = 1.77861                                                                          ν.sub.4 = 25.71                                 r.sub.8 = 1.0622                                                              t = 0.15      n.sub.t = 1.50                                                  f.sub.1 = 2.811                                                                             f.sub.2 = -52.675                                               f.sub.3 = 2.811                                                                             f.sub.4 = 1.485                                                 f = 1                                                                         Embodiment 13                                                                 NA = 0.45     WD = 0.518   β = -1/18.5                                   r.sub.1 = 2.1493                                                              d.sub.1 = 0.2304                                                                            n.sub.1 = 1.49020                                                                          ν.sub.1 = 57.8                                  r.sub.2 = -2.4314                                                             d.sub.2 = 0.0922                                                              r.sub.3 = -1.2120                                                             d.sub.3 = 0.2074                                                                            n.sub.2 = 1.49020                                                                          ν.sub.2 = 57.8                                  r.sub.4 = -2.9160                                                             d.sub.4 = 0.8296                                                              r.sub.5 = 2.1493                                                              d.sub.5 = 0.2304                                                                            n.sub.3 = 1.49020                                                                          ν.sub.3 = 57.8                                  r.sub.6 = -2.4314                                                             d.sub.6 = 0.0115                                                              r.sub.7 = 0.7141                                                              d.sub.7 = 0.1728                                                                            n.sub.4 = 1.77861                                                                          ν.sub.4 = 25.71                                 r.sub.8 = 1.6709                                                              t = 0.15      n.sub.t = 1.50                                                  f.sub.1 = 2.366                                                                             f.sub.2 =  -4.408                                               f.sub.3 = 2.366                                                                             f.sub.4 = 1.484                                                 f = 1                                                                         Embodiment 14                                                                 NA = 0.45     WD = 0.518   β = -1/18.5                                   r.sub.1 = 2.0987                                                              d.sub.1 = 0.2304                                                                            n.sub.1 = 1.49020                                                                          ν.sub.1 = 57.8                                  r.sub.2 = -2.7034                                                             d.sub.2 = 0.0922                                                              r.sub.3 = -1.2295                                                             d.sub.3 = 0.2074                                                                            n.sub.2 = 1.49020                                                                          ν.sub.2 = 57.8                                  r.sub.4 = -2.7034                                                             d.sub.4 = 0.8259                                                              r.sub.5 = 2.0987                                                              d.sub.5 = 0.2304                                                                            n.sub.3 = 1.49020                                                                          ν.sub.3 = 57.8                                  r.sub.6 = -2.7034                                                             d.sub.6 = 0.0115                                                              r.sub.7 = 0.7206                                                              d.sub.7 = 0.1728                                                                            n.sub.4 = 1.77861                                                                          ν.sub.4 = 25.71                                 r.sub.8 = 1.7740                                                              t = 0.15      n.sub.t = 1.50                                                  f.sub.1 = 2.449                                                                             f.sub.2  = -4.824                                               f.sub.3 = 2.449                                                                             f.sub.4 = 1.454                                                 f = 1                                                                         ______________________________________                                    

In the above-mentioned respective embodiments, reference symbols r₁through r₈ respectively represent radii of curvature of respective lenssurfaces, reference symbols d₁ through d₇ respectively representthicknesses of respective lenses and airspaces between respectivelenses, reference symbols n₁ through n₄ respectively representrefractive indices of respective lenses, reference symbols ν₁ through ν₄respectively represent Abbe's numbers of respective lenses, referencesymbol NA represents the numerical aperture, reference symbol WDrepresents the working distance (on the video disk side), referencesymbol β represent magnification, and reference symbol f represents thefocal length of the lens system as a whole. As respective embodiments ofthe present invention are designed on the basis that a monochromaticlight with the wavelength λ=6328 A is used, all the refractive indicesshown in the above are values at λ=6328 A.

Aberration curves of the above-mentioned respective embodiments areshown in FIGS. 2 through 15. As for astigmatism in these graphs ofaberration curves, Δm and Δs completely coincide with each other and,therefore, they are shown by the same curve.

Out of the above-mentioned embodiments, Embodiments 1 through 5 arearranged by using plastic lenses as all lenses which constitute the lenssystem while Embodiments 6 through 14 are arranged by using plasticlenses as the first, second and third lenses and optical glass for thefourth lens. As the fourth lens is to be exposed to the outside, it issometimes more preferable to use a glass material from the view point ofdurability by taking the external and operating conditions when usingthe objective into consideration. Besides, for Embodiments 8 through 14,plastic lenses having the same shape and data are used as the first andthird lenses and, therefore, these embodiments are advantageousespecially for mass production.

Moreover, plastic lenses bring forth the following advantages. Unlikeglass materials, plastic materials can be formed freely and, therefore,it is possible to intergrally form an annular projected portion alongthe circumference of each lens as shown by numerals 1, 2 and 3 inFIG. 1. When the lenses are formed as described in the above, it is notnecessary to insert a spacer ring between the first and second lensesfor the purpose of setting the airspace between them as it is evidentfrom FIG. 1. Furthermore, it is not necessary to use supporting ringsfor lenses which are otherwise required when mounting the lenses to thelens barrel. Consequently, the number of parts constituting theobjective is reduced. Therefore, the anti-vibration characteristic ofobjective is improved and it is possible to reduce the number of manhourfor production and cost of production.

For all optical plastic materials available at present, the refractiveindex is 1.6 or less and this imposes a considerable restriction on thedesign of lenses. In spite of such restriction, the present inventionprovides an objective for video disks which maintains high performanceequivalent to known objectives employing glass lenses and which isconsiderably lighter in weight.

I claim:
 1. A reproducing objective for video disks comprising a first,second, third and fourth lenses in the order from the reproducing lightsource side to the video disk side, said first lens being a positivelens, said second lens being a negative meniscus lens, said third lensbeing a positive lens, said fourth lens being a positive meniscus lens,at least one of said lenses being arranged as a plastic lens, saidreproducing objective for video disks fulfilling the followingconditions:(1) 2.1f<f₁ <3.2f (2) -55f<f₂ <-3.9f (3) 1.3f<f₄ <2.4f (4)f<r₈ <2.3f (5) d₂ <0.1f (6) 0.5f<d₄ wherein reference symbols f₁, f₂ andf₄ respectively represent focal lengths of the first, second and fourthlenses, reference symbol r₈ represents the radius of curvature ofsurface on the video disk side of the fourth lens, reference symbol d₂represents the airspace between the first and second lenses, referencesymbol d₄ represents the airspace between the second and third lenses,and reference symbol f represents the focal length of the lens system asa whole.
 2. A reproducing objective for video disks according to claim1, in which said reproducing objective for video disks has the followingnumerical data:

    ______________________________________                                        NA = 0.45    WD = 0.534    β = -1/18.5                                   r.sub.1 = 3.0032                                                              d.sub.1 = 0.2322                                                                           n.sub.1 = 1.59119                                                                           ν.sub.1 = 57.8                                  r.sub.2 = -2.6218                                                             d.sub.2 = 0.0580                                                              r.sub.3 = -1.1963                                                             d.sub.3 = 0.3250                                                                           n.sub.2 = 1.49819                                                                           ν.sub.2 = 57.8                                  r.sub.4 = -2.4894                                                             d.sub.4 = 0.7545                                                              r.sub.5 = 2.1883                                                              d.sub.5 = 0.2322                                                                           n.sub.3 = 1.59119                                                                           ν.sub.3 = 57.8                                  r.sub.6 = -3.1827                                                             d.sub.6 = 0.0116                                                              r.sub.7 = 0.7015                                                              d.sub.7 = 0.1857                                                                           n.sub.4 = 1.59119                                                                           ν.sub.4 = 57.8                                  r.sub.8 = 2.1531                                                              t = 0.15     n.sub.t = 1.50                                                   f.sub.1 = 2.405                                                                            f.sub.2 = -5.044                                                 f.sub.3 = 2.229                                                                            f.sub.4 = 1.680                                                  f = 1                                                                         ______________________________________                                    

wherein reference symbols r₁ through r₈ repsectively represent radii ofcurvature of respective lens surfaces, reference symbols d₁ through d₇respectively represent thicknesses of respective lenses and airspacesbetween respective lenses, reference symbols n₁, n₂, n₃ and n₄respectively represent refractive indices of respective lenses,reference symbols ν₁, 84 ₂, ν₃ and ν₄ respectively represent Abbe'snumbers of respective lenses at λ=6328 A, reference symbol t representsthe thickness of cover glass, reference symbol t_(n) represents therefractive index of cover glass, reference symbol NA represents thenumerical aperture, reference symbol WD represents the working distance,reference symbol β represents magnification, and reference symbol frepresents the focal length of the lens system as a whole.
 3. Areproducing objective for video disks according to claim 1, in whichsaid reproducing objective for video disks has the following numericaldata:

    ______________________________________                                        NA = 0.45    WD = 0.460    β =  -1/18.5                                  r.sub.1 = 2.0547                                                              d.sub.1 = 0.2300                                                                           n.sub.1 = 1.4902                                                                            ν.sub.1 = 57.8                                  r.sub.2 = 2.2681                                                              d.sub.2 = 0.0575                                                              r.sub.3 = -1.2145                                                             d.sub.3 = 0.3220                                                                           n.sub.2 = 1.4902                                                                            ν.sub.2 = 57.8                                  r.sub.4 = -2.2457                                                             d.sub.4 = 0.8165                                                              r.sub.5 = 1.2677                                                              d.sub.5 = 0.2300                                                                           n.sub.3 = 1.4902                                                                            ν.sub.3 = 57.8                                  r.sub.6 = -3.7974                                                             d.sub.6 = 0.0115                                                              r.sub.7 = 0.5727                                                              d.sub.7 = 0.1380                                                                           n.sub.4 = 1.4902                                                                            ν.sub.4 = 57.8                                  r.sub.8 = 1.7107                                                              t = 0.15     n.sub.t = 1.50                                                   f.sub.1 = 2.238                                                                            f.sub.2 = -6.013                                                 f.sub.3 = 1.968                                                                            f.sub.4 =  1.689                                                 f = 1                                                                         ______________________________________                                    

wherein reference symbols r₁ through r₈ respectively represent radii ofcurvature of respective lens surfaces, reference symbols d₁ through d₇respectively represent thicknesses of respective lenses and airspacesbetween respective lenses, reference symbols n₁, n₂, n₃ and n₄respectively represent refractive indices of respective lenses,reference symbols ν₁, ν₂, ν₃ and ν₄ respectively represent Abbe'snumbers of respective lenses at λ=6328 A, reference symbol t representsthe thickness of cover glass, reference symbol t_(n) represents therefractive index of cover glass, reference symbol NA represents thenumerical aperture, reference symbol WD represents the working distance,reference symbol β represents magnification, and reference symbol frepresents the focal length of the lens system as a whole.
 4. Areproducing objective for video disks according to claim 1, in whichsaid reproducing objective for video disks has the following numericaldata:

    ______________________________________                                        NA = 0.45    WD = 0.459    β =  -1/18.5                                  r.sub.1 = 2.3631                                                              d.sub.1 = 0.2296                                                                           n.sub.1 = 1.49819                                                                           ν.sub.1 = 57.8                                  r.sub.2 = -2.6102                                                             d.sub.2 = 0.0919                                                              r.sub.3 = -1.1435                                                             d.sub.3 = 0.3215                                                                           n.sub.2 = 1.49819                                                                           ν.sub.2 = 57.8                                  r.sub.4 = -1.7398                                                             d.sub.4 = 0.9186                                                              r.sub.5 = 1.1498                                                              d.sub.5 = 0.2067                                                                           n.sub.3 = 1.49819                                                                           ν.sub.3 = 57.8                                  r.sub.6 = -3.8750                                                             d.sub.6 = 0.0115                                                              r.sub.7 = 0.5679                                                              d.sub.7 = 0.1378                                                                           n.sub.4 = 1.49819                                                                           ν.sub.4 = 57.8                                  r.sub.8 = 1.4233                                                              t = 0.15     n.sub.t = 1.50                                                   f.sub.1 = 2.528                                                                            f.sub.2 = -8.160                                                 f.sub.3 = 1.805                                                                            f.sub.4 =  1.800                                                 f = 1                                                                         ______________________________________                                    

wherein reference symbols r₁ through r₈ respectively represent radii ofcurvature of respective lens surfaces, reference symbols d₁ through d₇respectively represent thicknesses of respective lenses and airspacesbetween respective lenses, reference symbols n₁, n₂, n₃ and n₄respectively represent indices of respective lenses, reference symbolsν₁, ν₂, ν₃ and ν₄ respectively represent Abbe's numbers of respectivelenses at λ=6328 A, reference symbol t represents the thickness of coverglass, reference symbol t_(n) represents the refractive index of coverglass, reference symbol NA represents the numerical aperture, referencesymbol WD represents the working distance, reference symbol β representsmagnification, and reference symbol f represents the focal length of thelens system as a whole.
 5. A reproducing objective for video disksaccording to claim 1, in which said reproducing objective for videodisks has the following numerical data:

    ______________________________________                                        NA = 0.45    WD = 0.460    β =  -1/18.5                                  r.sub.1 = 2.2811                                                              d.sub.1 = 0.2302                                                                           n.sub.1 = 1.49819                                                                           ν.sub.1 = 57.8                                  r.sub.2 = -2.6068                                                             d.sub.2 = 0.0575                                                              r.sub.3 = -1.1867                                                             d.sub.3 = 0.3222                                                                           n.sub.2 = 1.49819                                                                           ν.sub.2 = 57.8                                  r.sub.4 = -1.8606                                                             d.sub.4 = 0.8977                                                              r.sub.5 = 1.1534                                                              d.sub.5 = 0.2302                                                                           n.sub.3 = 1.49819                                                                           ν.sub.3 = 57.8                                  r.sub.6 = -3.9686                                                             d.sub.6 = 0.0115                                                              r.sub.7 = 0.5731                                                              d.sub.7 = 0.1381                                                                           n.sub.4 = 1.49819                                                                           ν.sub.4 = 57.8                                  r.sub.8 = 1.4170                                                              t = 0.15     n.sub.t = 1.50                                                   f.sub.1 = 2.481                                                                            f.sub.2 = -7.818                                                 f.sub.3 = 1.821                                                                            f.sub.4 =  1.832                                                 f = 1                                                                         ______________________________________                                    

wherein reference symbols r₁ through r₈ respectively represent radii ofcurvature of respective lens surfaces, reference symbols d₁ through d₇respectively represent thicknesses of respective lenses and airspacesbetween respective lenses, reference symbols n₁, n₂, n₃ and n₄respectively represent refractive indices of respective lenses,reference symbols ν₁, ν₂, ν₃ and ν₄ respectively represent Abbe'snumbers of respective lenses at λ=6328 A, reference symbol t representsthe thickness of cover glass, reference symbol t_(n) represents therefractive index of cover glass, reference symbol NA represents thenumerical aperture, reference symbol WD represents the working distance,reference symbol β represents magnification, and reference symbol frepresents the focal length of the lens system as a whole.
 6. Areproducing objective for video disks according to claim 1, in whichsaid reproducing objective for video disks has the following numericaldata:

    ______________________________________                                        NA = 0.45    WD = 0.460    β = -1/18.5                                   r.sub.1 = 2.3727                                                              d.sub.1 = 0.2298                                                                           n.sub.1 = 1.49819                                                                           ν.sub.1 = 57.8                                  r.sub.2 = -2.5701                                                             d.sub.2 = 0.0919                                                              r.sub.3 = -1.1415                                                             d.sub.3 = 0.3218                                                                           n.sub.2 = 1.49819                                                                           ν.sub.2 = 57.8                                  r.sub.4 = -1.7673                                                             d.sub.4 = 0.8963                                                              r.sub.5 = 1.1733                                                              d.sub.5 = 0.2298                                                                           n.sub.3 = 1.49819                                                                           ν.sub.3 = 57.8                                  r.sub.6 = -3.7409                                                             d.sub.6 = 0.0115                                                              r.sub.7 = 0.5716                                                              d.sub.7 = 0.1379                                                                           n.sub.4 = 1.49819                                                                           ν.sub.4 = 57.8                                  r.sub.8 = 1.4758                                                              t = 0.15     n.sub.t = 1.50                                                   f.sub.1 = 2.515                                                                            f.sub.2 = -7.805                                                 f.sub.3 = 1.821                                                                            f.sub.4 = 1.782                                                  f = 1                                                                         ______________________________________                                    

wherein reference symbols r₁ through r₈ respectively represent radii ofcurvature of respective lens surfaces, reference symbols d₁ through d₇respectively represent thicknesses of respective lenses and airspacesbetween respective lenses, reference symbols n₁, n₂, n₃ and n₄respectively represent refractive indices of respective lenses,reference symbols ν₁, ν₂, ν₃ and ν₄ respectively represent Abbe'snumbers of respective lenses at λ=6328 A, reference symbol t representsthe thickness of cover glass, reference symbol t_(n) represents therefractive index of cover glass, reference symbol NA represents thenumerical aperture, reference symbol WD represents the working distance,reference symbol β represents magnification, and reference symbol frepresents the focal length of the lens system as a whole.
 7. Areproducing objective for video disks according to claim 1, in whichsaid reproducing objective for video disks has the following numericaldata:

    ______________________________________                                        NA = 0.45    WD = 0.578    β = -1/18.5                                   r.sub.1 = 3.7269                                                              d.sub.1 = 0.2312                                                                           n.sub.1 = 1.49819                                                                           ν.sub.1 = 57.8                                  r.sub.2 = -2.3833                                                             d.sub.2 = 0.0925                                                              r.sub.3 = -1.0641                                                             d.sub.3 = 0.2081                                                                           n.sub.2 = 1.49819                                                                           ν.sub.2 = 57.8                                  r.sub.4 = -1.6543                                                             d.sub.4 = 0.9249                                                              r.sub.5 = 1.5397                                                              d.sub.5 = 0.2081                                                                           n.sub.3 = 1.49819                                                                           ν.sub.3 = 57.8                                  r.sub.6 = -3.1801                                                             d.sub.6 = 0.0116                                                              r.sub.7 = 0.7601                                                              d.sub.7 = 0.1387                                                                           n.sub.4 = 1.72309                                                                           ν.sub.4 = 28.46                                 r.sub.8 = 1.8463                                                              t = 0.15     n.sub.t = 1.50                                                   f.sub.1 = 2.955                                                                            f.sub.2 = -6.782                                                 f.sub.2 = 2.113                                                                            f.sub.4 = 1.696                                                  f = 1                                                                         ______________________________________                                    

wherein reference symbols r₁ through r₈ respectively represent radii ofcurvature of respective lens surfaces, reference symbols d₁ through d₇respectively represent thicknesses of respective lenses and airspacesbetween respective lenses, reference symbols n₁, n₂, n₃ and n₄respectively represent refractive indices of respective lenses,reference symbols ν₁, ν₂, ν₃ and ν₄ respectively represent Abbe'snumbers of respective lenses at λ=6328 A, reference symbol t representsthe thickness of cover glass, reference symbol t_(n) represents therefractive index of cover glass, reference symbol NA represents thenumerical aperture, reference symbol WD represents the working distance,reference symbol β represents magnification, and reference symbol frepresents the focal length of the lens system as a whole.
 8. Areproducing objective for video disks according to claim 1, in whichsaid reproducing objective for video disks has the following numericaldata:

    ______________________________________                                        NA = 0.45    WD = 0.580    β =  -1/18.5                                  r.sub.1 = 3.9264                                                              d.sub.1 = 0.2321                                                                           n.sub.1 = 1.49819                                                                           ν.sub.1 = 57.8                                  r.sub.2 = -2.4314                                                             d.sub.2 = 0.0928                                                              r.sub.3 = -1.0479                                                             d.sub.3 = 0.2089                                                                           n.sub.2 = 1.49819                                                                           ν.sub.2 = 57.8                                  r.sub.4 = -1.6446                                                             d.sub.4 = 0.8704                                                              r.sub.5 = 1.6841                                                              d.sub.5 = 0.2321                                                                           n.sub.3 = 1.49819                                                                           ν.sub.3 = 57.8                                  r.sub.6 = -2.9405                                                             d.sub.6 = 0.116                                                               r.sub.7 = 0.8141                                                              d.sub.7 = 0.1741                                                                           n.sub.4 = 1.77861                                                                           ν.sub.4 = 25.71                                 r.sub.8 = 1.8989                                                              t = 0.15     n.sub.t = 1.50                                                   f.sub.1 = 3.051                                                                            f.sub.2 = -6.561                                                 f.sub.3 = 2.186                                                                            f.sub.4 =  1.710                                                 f = 1                                                                         ______________________________________                                    

wherein reference symbols r₁ through r₈ respectively represent radii ofcurvature of respective lens surfaces, reference symbols d₁ through d₇respectively represent thicknesses of respective lenses and airspacesbetween respective lenses, reference symbols n₁, n₂, n₃ and n₄respectively represent refractive indices of respective lenses,reference symbols ν₁, ν₂, ν₃ and ν₄ respectively represent Abbe'snumbers of respective lenses at λ=6328 A, reference symbol t representsthe thickness of cover glass, reference symbol t_(n) represents therefractive index of cover glass, reference symbol NA represents thenumerical aperture, reference symbol WD represents the working distance,reference symbol β represents magnification, and reference symbol frepresents the focal length of the lens system as a whole.
 9. Areproducing objective for video disks according to claim 1, in whichsaid reproducing objective for video disks has the following numericaldata:

    ______________________________________                                        NA = 0.45    WD = 0.530    β =  -1/18.5                                  r.sub.1 = 2.7135                                                              d.sub.1 = 0.2304                                                                           n.sub.1 = 1.49819                                                                           ν.sub.1 = 57.8                                  r.sub.2 = -2.3475                                                             d.sub.2 = 0.0922                                                              r.sub.3 = -1.1732                                                             d.sub.3 = 0.2074                                                                           n.sub.2 = 1.49819                                                                           ν.sub.2 = 57.8                                  r.sub.4 = -2.2729                                                             d.sub.4 = 0.9102                                                              r.sub.5 = 2.3475                                                              d.sub.5 = 0.2304                                                                           n.sub.3 = 1.49819                                                                           ν.sub.3 = 57.8                                  r.sub.6 = -2.7135                                                             d.sub.6 = 0.0115                                                              r.sub.7 = 0.7232                                                              d.sub.7 = 0.1728                                                                           n.sub.4 = 1.77861                                                                           ν.sub.4 = 25.71                                 r.sub.8 = 1.8951                                                              t = 0.15     n.sub.t = 1.50                                                   f.sub.1 = 2.565                                                                            f.sub.2 = -5.193                                                 f.sub.3 = 2.565                                                                            f.sub.4 =  1.411                                                 f = 1                                                                         ______________________________________                                    

wherein reference symbols r₁ through r₈ respectively represent radii ofcurvature of respective lens surfaces, reference symbols d₁ through d₇respectively represent thicknesses of respective lenses and airspacesbetween respective lenses, reference symbols n₁, n₂, n₃ and n₄respectively represent refractive indices of respective lenses,reference symbols ν₁, ν₂, ν₃ and ν₄ respectively represent Abbe'snumbers of respective lenses at λ=6328 A, reference symbol t representsthe thickness of cover glass, reference symbol t_(n) represents therefractive index of cover glass, reference symbol NA represents thenumerical aperture, reference symbol WD represents the workdingdistance, reference symbol β represents magnification, and referencesymbol f represents the focal length of the lens system as a whole. 10.A reproducing objective for video disks according to claim 1, in whichsaid reproducing objective for video disks has the following numericaldata:

    ______________________________________                                        NA = 0.45    WD = 0.531    β =  -1/18.5                                  r.sub.1 = 2.4705                                                              d.sub.1 = 0.2306                                                                           n.sub.1 = 1.49819                                                                           ν.sub.1 = 57.8                                  r.sub.2 = -2.3810                                                             d.sub.2 = 0.0922                                                              r.sub.3 = -1.1846                                                             d.sub.3 = 0.2075                                                                           n.sub.2 = 1.49819                                                                           ν.sub.2 = 57.8                                  r.sub.4 = -2.4965                                                             d.sub.4 = 0.8647                                                              r.sub.5 = 2.3810                                                              d.sub.5 = 0.2306                                                                           n.sub.3 = 1.49819                                                                           ν.sub.3 = 57.8                                  r.sub.6 = -2.4705                                                             d.sub.6 = 0.0115                                                              r.sub.7 = 0.7225                                                              d.sub.7 = 0.1729                                                                           n.sub.4 = 1.77861                                                                           ν.sub.4 = 25.71                                 r.sub.8 = 1.7879                                                              t = 0.15     n.sub.t = 1.50                                                   f.sub.1 = 2.473                                                                            f.sub.2 = 4.776                                                  f.sub.3 = 2.473                                                                            f.sub.4 = 1.454                                                  f = 1                                                                         ______________________________________                                    

wherein reference symbols r₁ through r₈ respectively represent radii ofcurvature of respective lens surfaces, reference symbols d₁ through d₇respectively represent thicknesses of respective lenses and airspacesbetween respective lenses, reference symbols n₁, n₂, n₃ and n₄respectively represent refractive indices of respective lenses,reference symbols ν₁, ν₂, ν₃ and ν₄ respectively represent Abbe'snumbers of respective lenses at λ=6328 A, reference symbol t representsthe thickness of cover glass, reference symbol t_(n) represents therefractive index of cover glass, reference symbol NA represents thenumerical aperture, reference symbol WD represents the working distance,reference symbol β represents magnification, and reference symbol frepresents the focal length of the lens system as a whole.
 11. Areproducing objective for video disks according to claim 1, in whichsaid reproducing objective for video disks has the following numericaldata:

    ______________________________________                                        NA = 0.45    WD = 0.531    β = -1/18.5                                   r.sub.1 = 2.3500                                                              d.sub.1 = 0.2306                                                                           n.sub.1 = 1.49819                                                                           ν.sub.1 = 57.8                                  r.sub.2 = -2.4361                                                             d.sub.2 = 0.0923                                                              r.sub.3 = -1.1911                                                             d.sub.3 = 0.2076                                                                           n.sub.2 = 1.49819                                                                           ν.sub.2 = 57.8                                  r.sub.4 = -2.6000                                                             d.sub.4 = 0.8418                                                              r.sub.5 = 2.3500                                                              d.sub.5 = 0.2306                                                                           n.sub.3 = 1.49819                                                                           ν.sub.3 = 57.8                                  r.sub.6 = -2.4361                                                             d.sub.6 = 0.0115                                                              r.sub.7 = 0.7255                                                              d.sub.7 = 0.1730                                                                           n.sub.4 = 1.77861                                                                           ν.sub.4 = 25.71                                 r.sub.8 = 1.7739                                                              t = 0.15     n.sub.t = 1.50                                                   f.sub.1 = 2.440                                                                            f.sub.2 = -4.639                                                 f.sub.3 = 2.440                                                                            f.sub.4 = 1.470                                                  f = 1                                                                         ______________________________________                                    

wherein reference symbols r₁ through r₈ respectively represent radii ofcurvature of respective lens surfaces, reference symbols d₁ through d₇respectively represent thicknesses of respective lenses and airspacesbetween respective lenses, reference symbols n₁, n₂, n₃ and n₄respectively represent refractive indices of respective lenses,reference symbols ν₁, ν₂, ν₃ and ν₄ respectively represent Abbe'snumbers of respective lenses at λ=6328 A, reference symbol t representsthe thickness of cover glass, reference symbol t_(n) represents therefractive index of cover glass, reference symbol NA represents thenumerical aperture, reference symbol WD represents the working distance,reference symbol β represents magnification, and reference symbol frepresents the focal length of the lens system as a whole.
 12. Areproducing objective for video disks according to claim 1, in whichsaid reproducing objective for video disks has the following numericaldata:

    ______________________________________                                        NA = 0.45    WD = 0.519    β =  -1/18.5                                  r.sub.1 = 2.0414                                                              d.sub.1 = 0.2306                                                                           n.sub.1 = 1.49819                                                                           ν.sub.1 = 57.8                                  r.sub.2 = -2.4915                                                             d.sub.2 = 0.0922                                                              r.sub.3 = -1.2311                                                             d.sub.3 = 0.2079                                                                           n.sub.2 = 1.49819                                                                           ν.sub.2 = 57.8                                  r.sub.4 = -3.2939                                                             d.sub.4 = 0.7955                                                              r.sub.5 = 2.0414                                                              d.sub.5 = 0.2306                                                                           n.sub.3 = 1.49819                                                                           ν.sub.3 = 57.8                                  r.sub.6 = -2.4915                                                             d.sub.6 = 0.0115                                                              r.sub.7 = 0.7239                                                              d.sub.7 = 0.1729                                                                           n.sub.4 = 1.77861                                                                           ν.sub.4 = 25.71                                 r.sub.8 = 1.6673                                                              t = 0.15     n.sub.t = 1.50                                                   f.sub.1 = 2.291                                                                            f.sub.2 = -4.083                                                 f.sub.3 = 2.291                                                                            f.sub.4 =  1.521                                                 f = 1                                                                         ______________________________________                                    

wherein reference symbols r₁ through r₈ respectively represent radii ofcurvature of respective lens surfaces, reference symbols d₁ through d₇respectively represent thicknesses of respective lenses and airspacesbetween respective lenses, reference symbols n₁, n₂, n₃ and n₄respectively represent refractive indices of respective lenses,reference symbols ν₁, ν₂, ν₃ and ν₄ respectively represent Abbe'snumbers of respective lenses at λ=6328 A, reference symbol t representsthe thickness of cover glass, reference symbol t_(n) represents therefractive index of cover glass, reference symbol NA represents thenumerical aperture, reference symbol WD represents the working distance,reference symbol β represents magnification, and reference symbol frepresents the focal length of the lens system as a whole.
 13. Areproducing objective for video disks according to claim 1, in whichsaid reproducing objective for video disks has the following numericaldata:

    ______________________________________                                        NA = 0.45    WD = 0.466    β =  -1/18.5                                  r.sub.1 = 1.4004                                                              d.sub.1 = 0.3362                                                                           n.sub.1 = 1.49819                                                                           ν.sub.1 = 57.8                                  r.sub.2 = ∞                                                             d.sub.2 = 0.0815                                                              r.sub.3 = -1.1768                                                             d.sub.3 = 0.6990                                                                           n.sub.2 = 1.49819                                                                           ν.sub.2 = 57.8                                  r.sub.4 = -1.4754                                                             d.sub.4 = 0.5296                                                              r.sub.5 = 1.4004                                                              d.sub.5 = 0.3362                                                                           n.sub.3 = 1.49819                                                                           ν.sub.3 = 57.8                                  r.sub.6 = ∞                                                             d.sub.6 = 0.0125                                                              r.sub.7 = 0.5856                                                              d.sub.7 = 0.1398                                                                           n.sub.4 = 1.77861                                                                           ν.sub.4 = 25.71                                 r.sub.8 = 1.0622                                                              t = 0.15     n.sub.t = 1.50                                                   f.sub.1 = 2.811                                                                            f.sub.2 = -52.675                                                f.sub.3 = 2.811                                                                            f.sub.4 =  1.485                                                 f = 1                                                                         ______________________________________                                    

wherein reference symbols r₁ through r₈ respectively represent radii ofcurvature of respective lens surfaces, reference symbols d₁ through d₇respectively represent thicknesses of respective lenses and airspacesbetween respective lenses, reference symbols n₁, n₂, n₃ and n₄respectively represent refractive indices of respective lenses,reference symbols ν₁, ν₂, ν₃ and ν₄ respectively represent Abbe'snumbers of respective lenses at λ=6328 A, reference symbol t representsthe thickness of cover glass, reference symbol t_(n) represents therefractive index of cover glass, reference symbol NA represents thenumerical aperture, reference symbol WD represents the working distance,reference symbol β represents magnification, and reference symbol frepresents the focal length of the lens system as a whole.
 14. Areproducing objective for video disks according to claim 1, in whichsaid reproducing objective for video disks has the following numericaldata:

    ______________________________________                                        NA = 0.45    WD = 0.518    β = -1/18.5                                   r.sub.1 = 2.1493                                                              d.sub.1 = 0.2304                                                                           n.sub.1 = 1.49020                                                                           ν.sub.1 = 57.8                                  r.sub.2 = -2.4314                                                             d.sub.2 = 0.0922                                                              r.sub.3 = -1.2120                                                             d.sub.3 = 0.2074                                                                           n.sub.2 = 1.49020                                                                           ν.sub.2 = 57.8                                  r.sub.4 = -2.9160                                                             d.sub.4 = 0.8296                                                              r.sub.5 = 2.1493                                                              d.sub.5 = 0.2304                                                                           n.sub.3 = 1.49020                                                                           ν.sub.3 = 57.8                                  r.sub.6 = -2.4314                                                             d.sub.6 = 0.0115                                                              r.sub.7 = 0.7141                                                              d.sub.7 = 0.1728                                                                           n.sub.4 = 1.77861                                                                           ν.sub.4 = 25.71                                 r.sub.8 = 1.6709                                                              t = 0.15     n.sub.t = 1.50                                                   f.sub.1 = 2.366                                                                            f.sub.2 = -4.408                                                 f.sub.3 = 2.366                                                                            f.sub.4 = 1.484                                                  f = 1                                                                         ______________________________________                                    

wherein reference symbols r₁ through r₈ respectively represent radii ofcurvature of respective lens surfaces, reference symbols d₁ through d₇respectively represent thicknesses of respective lenses and airspacesbetween respective lenses, reference symbols n₁, n₂, n₃ and n₄respectively represent refractive indices of respective lenses,reference symbols ν₁, ν₂, ν₃ and ν₄ respectively represent Abbe'snumbers of respective lenses at λ=6328 A, reference symbol t representsthe thickness of cover glass, reference symbol t_(n) represents therefractive index of cover glass, reference symbol NA represents thenumerical aperture, reference symbol WD represents the working distance,reference symbol β represents magnification, and reference symbol frepresents the focal length of the lens system as a whole.
 15. Areproducing objective for video disks according to claim 1, in whichsaid reproducing objective for video disks has the following numericaldata:

    ______________________________________                                        NA = 0.45    WD = 0.518    β = -1/18.5                                   r.sub.1 = 2.0987                                                              d.sub.1 = 0.2304                                                                           n.sub.1 = 1.49020                                                                           ν.sub.1 = 57.8                                  r.sub.2 = -2.7034                                                             d.sub.2 = 0.0922                                                              r.sub.3 = -1.2295                                                             d.sub.3 = 0.2074                                                                           n.sub.2 = 1.49020                                                                           ν.sub.2 = 57.8                                  r.sub.4 = -2.7034                                                             d.sub.4 = 0.8259                                                              r.sub.5 = 2.0987                                                              d.sub.5 = 0.2304                                                                           n.sub.3 = 1.49020                                                                           ν.sub.3 = 57.8                                  r.sub.6 = -2.7034                                                             d.sub.6 = 0.0115                                                                           r.sub.7 = 0.7206                                                 d.sub.7 = 0.1728                                                                           n.sub.4 = 1.77861                                                                           ν.sub.4 = 25.71                                 r.sub.8 = 1.7740                                                              t = 0.15     n.sub.t = 1.50                                                   f.sub.1 = 2.449                                                                            f.sub.2 = -4.824                                                 f.sub.3 = 2.449                                                                            f.sub.4 = 1.454                                                  f = 1                                                                         ______________________________________                                    

wherein reference symbols r₁ through r₈ respectively represent radii ofcurvature of respective lens surfaces, reference symbols d₁ through d₇respectively represent thicknesses of respective lenses and airspacesbetween respective lenses, reference symbols n₁, n₂, n₃ and n₄respectively represent refractive indices of respective lenses,reference symbols ν₁, ν₂, ν₃ and ν₄ respectively represent Abbe'snumbers of respective lenses at λ=6328 A, reference symbol t representsthe thickness of cover glass, reference symbol t_(n) represents therefractive index of cover glass, reference symbol NA represents thenumerical aperture, reference symbol WD represents the working distance,reference symbol β represents magnification, and reference symbol frepresents the focal length of the lens system as a whole.